1. Field of Technology
The present disclosure relates to an austenitic stainless steel. In particular, the disclosure relates to a cost-effective austenitic stainless steel composition having low nickel and low molybdenum yet having improved corrosion resistance and comparable formability properties compared to certain alloys containing higher nickel and molybdenum.
2. Description of the Background of the Technology
Austenitic stainless steels exhibit a combination of highly desirable properties that make them useful for a wide variety of industrial applications. These steels possess a base composition of iron that is balanced by the addition of austenite-promoting and stabilizing elements, such as nickel, manganese, and nitrogen, to allow additions of ferrite-promoting elements, such as chromium and molybdenum, which enhance corrosion resistance, to be made while maintaining an austenitic structure at room temperature. The austenitic structure provides the steel with highly desirable mechanical properties, particularly toughness, ductility, and formability.
An example of an austenitic stainless steel is EN 1.4432 stainless steel, which is a 16.5-18.5% chromium, 10.5-13% nickel, and 2.5-3.0% molybdenum-containing alloy. The ranges of alloying ingredients in this alloy are maintained within the specified ranges in order to maintain a stable austenitic structure. As is understood by one skilled in the art, nickel, manganese, copper, and nitrogen content, for example, contribute to the stability of the austenitic structure. However, the rising costs of nickel and molybdenum have created the need for cost-effective alternatives to EN 1.4432 that still exhibit high corrosion resistance and good formability. Recently, lean duplex alloys such as UNS S32003 (AL 2003™ alloy) have been used as lower-cost alternatives to EN 1.4432, but while these alloys have good corrosion resistance, they contain approximately 50% ferrite, which gives them higher strength and lower ductility than EN 1.4432, and as a consequence, they are not as formable. Duplex stainless steels are also more limited in use for both high and low temperatures, as compared to EN 1.4432.
Another austenitic alloy is Grade 317 (UNS S31700). S31700 contains 18.0-20.0% chromium, 11.0-15.0% nickel, and 3.0-4.0% molybdenum. Because of its higher Ni and Mo content, S31700 is a more costly alternative to EN 1.4432 and another commonly used austenitic grade, Type 316 (UNS S31600), which contains 16.0-18.0 chromium, 10.0-14.0% nickel, and 2.0-3.0% molybdenum. While the corrosion resistance of S31700 is superior to that of EN 1.4432 and S31600, its higher-cost raw materials make the use of S31700 too costly for many applications.
Another alloy alternative is Grade 216 (UNS S21600), which is described in U.S. Pat. No. 3,171,738. S21600 contains 17.5-22% chromium, 5-7% nickel, 7.5-9% manganese, 2-3% molybdenum, and 0.25-0.50 nitrogen. S21600 is a lower nickel, higher manganese variant of S31600 that contains very high nitrogen, which gives it greater strength and improves corrosion resistance. However, the formability of S21600 is not as good as that of S31600 or EN 1.4432, and the very low ferrite number of S21600 (−6.2) makes casting and welding more difficult. Also, because S21600 contains a similar amount of molybdenum as does EN 1.4432, switching to S21600 provides no cost savings for molybdenum.
Other examples of austenitic stainless steels include numerous alloys in which nickel is replaced with manganese to maintain an austenitic structure, such as is practiced with Type 201 steel (UNS S20100) and similar grades. However, although Type 201 steel is a low-nickel alloy having good corrosion resistance, it has poor formability properties. There is a need to be able to produce an alloy having corrosion resistance and formability as good as or better than those of EN 1.4432, while containing lower amounts of nickel and molybdenum, so as to be cost-effective. Furthermore, there is a need for such an alloy to have, unlike duplex alloys, a temperature application range comparable to that of standard austenitic stainless steels, for example from cryogenic temperatures up to 1000° F.
Accordingly, the present invention provides a solution that is not currently available in the marketplace, which is a formable austenitic stainless steel alloy composition that has corrosion resistance properties as good as or superior to those of EN 1.4432 but provides raw material cost savings. Accordingly, the invention is an austenitic alloy that uses a combination of the elements Mn, Cu, and N, to replace Ni and Mo in a manner to create an alloy with comparable or superior corrosion resistance, formability, and other properties relative to certain higher nickel and molybdenum alloys at a significantly lower raw material cost. Optionally, the elements W and Co may be used independently or in combination to replace the elements Mo and Ni, respectively.